Apparatus for generating coded alternating current



April 19, 1955 A. J. SORENSEN 2,706,813

APPARATUS FOR GENERATING CODED ALTERNATING CURRENT Filed July 25, 1952 'Tfr Governed b A 25 5 Tmiiz'c 00n iiii0n$ in Advance LgVgENTOR.

Andrew ol'ezzsen 117L914. 7 w.k.$t-i'.

H15 ATTORNEY United States Patent OfiFice 2,706,813 Patented Apr. 19, 1955 APPARATUS FOR GENERATING CODED ALTERNATING CURRENT Andrew J. Sorensen, Edgewood, Pa., assignor to Westinghouse Air Brake Company, Wilmer-ding, Pa., a corporation of Pennsylvania Application July 23, 1952, Serial No. 300,554

6 Claims. (Cl. 340-345) My invention relates to apparatus for generating coded alternating current, and more particularly to apparatus for deriving coded alternating current from a source of unidirectional current.

Coded alternating current is used quite extensively in railway signaling systems. In a typical system employing such current, a stretch of railway track is divided into successive insulated track sections, each of which is provided with a track circuit, and the coded alternating current is supplied to the track circuits for the control of cab signals, or wayside signals, or both. The alternating current is coded at different code rates according to different traffic conditions. Alternating current having a frequency of 100 cycles per second is commonly used to avoid interference from commercial power lines and the like, and the coding is effected by coders which periodically interrupt the circuit at the desired rate, which in the case of the usual three indication systems is 180 and 75 times per minute. When the 180 code is supplied to a track circuit, it indicates a clear track in the section ahead and the signal responsive to the code will be green so that the engineer of a train approaching the signal may proceed at high speed. When the 75 code is supplied to a track circuit, it indicates that there is a train one section in advance of the section occupied by the train and the engineer of the train viewing the signal controlled by the 75 code which, under these conditions would be yellow, should proceed at medium speed. When steady energy or no energy is supplied to a track section in such a system, the signal for that section will be red which would indicate stop, and a train approaching the signal should come to a halt.

Each such code consists of alternate on and off periods, current flowing in each on period and no current flowing in each off period. Preferably, the on and off periods of a code are of substantially equal duration. Generally speaking, the alternating current is supplied to the different track circuits from a transmission line extending along the railway from a power plant at which the generator is located. The coding however is usually effected by coders individual to each track circuit.

It has been proposed to generate alternating current for railway signal systems by generators located along the railway, one for each control track circuit. By using such an arrangement, the signaling system is not subject to failure due to storms and other weather conditions which may damage a transmission line and put it out of service. Also, such an arrangement avoids the relatively high cost of installing and maintaining a transmission line. When a generator is used for each track circuit, the driving energy is derived from a local battery. Such local generators must therefore be reliable in operation, relatively low in cost, and relatively high in efficiency in order to save on the energy output required from the battery.

One means for generating alternating current locally is a tuned circuit comprising a capacitor and a coil wound on an iron core, the coil often being a winding of a transformer. The tuned circuit is periodically charged by a local battery and during the periods when the tuned circuit is not receiving charging energy, it is permitted to oscillate at a preselected frequency determined by the proportioning of the parts. Certain difliculties have sometimes been encountered with this type of generating equipment. When the charging circuit is closed, the magnetic flux then linking the coil may be in phase or out of phase with the flux built up by the battery current. It the fluxes are in phase, the charging current may be great thus causing an unduly heavy drain on the battery and also causing a large initial amplitude of the next series of oscillations. If the fluxes are in phase opposition, then the charging current may be small and the amplitude of the oscillations may be small also. This possibility of erratic operation has proved troublesome at times.

One object of my invention is to provide a novel and improved apparatus for generating coded alternating current from power derived from a source of direct current.

Another object of my invention is to provide in apparatus of the type here described, novel, reliable and inexpensive means for supplying coded alternating current of a preselected frequency and code rate from power derived from a source of direct current.

A further object of my invention is to provide, in apparatus for a railway signaling system, a novel and improved means for supplying a track circuit with coded alternating current from power derived from a local battery.

Still another object of my invention is to provide a novel means for deriving a wave or code impulse of alternating current of a preselected duration from a wave of damped oscillations.

Yet another object of my invention is to provide a novel means for deriving a substantially identical wave form from my apparatus for generating alternating current each time the apparatus operates.

According to my invention, I provide a coding means, an oscillating circuit and a suitable source of direct current. The coding means actuates a code-following device, which in turn controls a suitable repeater which repeats the action of the code-following device during onehalf of its cyclic operation but which slightly lags the code-following device during the other half. The re peater controls the energization of the oscillating circuit which comprises a capacitor and a winding which is mounted on a magnetizable core. When the oscillating circuit is energized, energy is stored in the magnetizable core and when the energy supply is cut off, the stored energy is dissipated in oscillations having a damped wave form. The frequency of the oscillations and their initial amplitude may be controlled by proportioning the parts. The oscillations induce an electromotive force in the winding and the winding is coupled by suitable means to a control circuit, the control circuit being governed by the code-following device. The coupling means may be a transformer and if his, the primary winding of the transformer may be the winding used in the oscillating circuit.

My invention also includes means for rapidly damping oscillations in the oscillating circuit in order to achieve substantially similar magnetic conditions in the magnetizable core during each cycle of operation. I have achieved this by either short circuiting the winding in the oscillating circuit near the end of each series of oscillations or by short circuiting a special damping winding which is closely magnetically coupled with the winding in the oscillating circuit. Throughout one-half of their cyclic operation, the code-following means and its repeater are in phase and throughout the other half, the repeater lags the code-following means, and the circuits are so arranged that the winding in the oscillating circuit is short-circuited prior to its receiving energy from the direct current source. This short-circuiting of the winding puts the magnetic core in approximately the same magnetic condition each time the oscillating circuit is charged, and helps to eliminate erratic operation due to residual fiux in the magnetizable core affecting the amount .of charging current and the initial amplitude of each series of oscillations.

Other objects of my invention will appear hereinafter as the characteristic features of construction and mode of operation of apparatus embodying my invention are described in detail.

I shall describe two forms of apparatus embodying my invention and shall then point out the novel features thereof in claims.

In the accompanying drawings,

Fig. l is a diagrammatic view showing the apparatus embodying my invention when used to supply coded alternating current to the track circuit of a railway signaling system.

Fig. 2 is a diagrammatic view showing a train and traincarried cab signaling apparatus which may be operated by the apparatus shown in Fig. 1.

Fig. 3 is a diagrammatic view showing a modification of the apparatus embodying my invention for deriving coded alternating current from direct current.

Figs. 4 and 5 are diagrams illustrating operating characteristics of the apparatus of Figs. 1 and 3.

In each of the dilferent views, similar reference characters are used to designate similar parts.

Referring to Fig. l, a stretch of track made up of track rails 5 and 6 is divided into track sections 1T, 2T and 3T, by insulated joints 7. Traffic normally moves through this stretch in the direction indicated by the arrow, that is, from left to right.

Each track section is provided with a coded track circuit. Because these track circuits are substantially identical, I shall describe only the coded track circuit associated with section 2T. Energized in multiple over an obvious circuit are code transmitters ISBCT and '75CT which open and close their contacts 180 times per minute and 75 times per minute, respectively. As was mentioned earlier, the 180 code rate is indicative of a clear track, and the 75 code rate is indicative of a trafiic condition requiring the train to proceed with caution.

A signal control relay 3HR is essentially a repeater of a track relay (not shown) for section 3T, and hence it is controlled by trafiic conditions in track section 3T, that is, the section in advance of section 2T. If relay 3HR is picked up, showing that section 31" is clear, a code transmitting relay 2CTR is periodically energized over a circuit which may be traced from the positive terminal B of a local battery LB, over front contact a of code transmitter 180CT, front contact a of relay 3I-IR, and the winding of relay ZCTR to the negative terminal N of battery LB. Since the code transmitter 180CT operates its contact a 180 times per minute, it follows that when section ST is unoccupied relay ZCTR will open and close its contacts 180 times per minute.

If there is a train in section 3T, relay SHR will be released, thus opening the previously traced circuit and closing a circuit which can be traced from the positive terminal B of battery LB, over front contact a of code transmitter 75CT, back contact a of relay SHR, and the winding of code transmitter relay 2CTR to negative terminal N of battery LB. It will be seen, therefore, that when section ST is occupied by a train, the contacts of relay ZCTR will operate 75 times per minute.

Assuming track sections 2T and 3T are unoccupied, signal control relay 3HR will be picked up, and code transmitting relay 2CTR will be energized 180 times per minute for the reasons just described. Under these conditions, track relay ZTR will be supplied with direct current coded at a 180 code rate over a circuit which may be traced from the positive terminal of a track battery 2TB, over back contact c of relay ZCT R, rail 5, the winding of relay 2TR, and rail 6 to the negative terminal of battery 2TB.

With the relay ZTR operating its contacts 180 times per minute, contact a of relay ZTR alternately energizes the upper and lower portions of the primary winding P of a transformer 2DT over an obvious circuit. An alternating current is thereby induced in the secondary windings of transformer ZDT which current is mechanically rectified by contact b of relay ZTR and is supplied to the winding of a slow pick-up slow-release signal control relay 2HR, to thereby energize that relay. Winding P of transformer 2DT is also connected as an auto-transformer to a tuned decoding unit 180DU of well known form and so constructed that it will supply sufficient energy to pick up a slow-release control relay 213R when and only when it is energized with energy coded at 180 times per minute. With 180 code rate being supplied, a green lamp G at signal will be energized over a circuit running from the positive terminal B of battery LB, over front contact a of relay 2I-IR, front contact a of relay 2DR, and the filament of lamp G to the negative terminal N of battery LB.

I will now assume that section ST is occupied and section 2T is unoccupied. Under these conditions signal control relay 3HR will be released and will cause relay 2CTR to operate its contact 0 at a code rate so that track relay ZTR will be energized by 75 code energy. Due to the construction of the decoding unit DU, insufficient energy will be supplied to control relay 2DR under these conditions to keep the relay 2DR picked up, and it will therefore release. However, signal control relay ZHR will remain picked up. Yellow lamp Y of signal 28 will therefore now become energized over a circuit which can be traced from the positive terminal B of battery LB, over front contact a of relay ZHR, back contact a of relay 2DR, and the filament of yellow lamp Y to the negative terminal N of battery LB.

I will now assume that with section 3T occupied and section 2T unoccupied, a train or vehicle enters section 2T. Under these conditions, the wheels and axles of the train or vehicle will shunt track relay ZTR causing it to release and stay released. Hence, no energy will be supplied to relays ZHR and ZDR and they will both release. This will cause the red lamp R of signal 25 to become energized over a circuit which can be traced from the positive terminal B of battery LB, over back contact a of relay ZHR, and the filament of the red lamp R to the negative terminal N of battery LB, so that signal 28 will now indicate stop.

A train moving through the stretch of track may be equipped with cab signal apparatus. When this is the case, in addition to the coded direct current which is supplied to the track circuit for controlling the wayside signal 25, coded alternating current will also be supplied to the rails to control the cab signal by means to be described presently. The cab signaling apparatus includes a receiver which is mounted in an inductive relationship to the rails. The coded alternating current supplied to the rails induces an alternating electromotive force in the receiver which is used to operate a train-carried codefollowing relay, which in turn governs train-carried controlling devices or train-carried indication means through decoding apparatus that is selectively responsive to the code rate at which the code-following relay is operating. Normally, the train-carried indication means provides the same indication on a train that is displayed by the wayside signal. It should be noted that when no coded alternating current energy is being received by the receiver, the train-carried controlling devices or indication means operate to their most restrictive condition. Such train-carried signaling systems are well known in the art, and a detailed description of them is therefore believed to be unnecessary. One form of cab signaling system which will operate in the manner just described is described in Letters Patent of the United States No. 1,986,679, issued January 1, 1935, to L. V. Lewis.

Cab signaling equipment of the type described above is shown diagrammatically in Fig. 2 in the drawings. It will be seen that the receiver coils RC are mounted on the train in inductive relationship to the rails, and the electromotive force induced in them is supplied to code responsive apparatus. When the code rate of the received electromotive force is 180 times per minute, energy is supplied from the code responsive apparatus to the windings of both relays RA and RB, and they are energized to pick up their contacts as shown on the drawings. Accordingly, the green lamp G of the cab signal indicator CS is illuminated due to energy flowing in the circuit which may be traced from the positive terminal of a battery CB, over front contact a of relay RA, front contact a of relay RB, and the filament of lamp G to the negative terminal of battery CB. When the electromotive force induced in the receiver coil is at a code rate of 75 times per minute, relay RB releases its contacts and the yellow lamp Y is illuminated by a circuit which may be traced from the positive terminal of battery CB, over front contact a of relay RA, back contact a of relay RB, and the filament of the yellow lamp Y of cab signal OS to the negative terminal of battery CB. When no coded alternating current energy is received by the receiving coils, both relays RA and RB release and the red lamp R of cab signal CS is illuminated by a circuit which may be traced from the positive ter minal of battery CB through back contact a of relay RA, and the filament of red lamp R of cab signal CS to the negative terminal of battery CB.

In accordance with my present invention, the coded alternating current for operating the cab signal in the manner just described is supplied by an oscillating circuit. This oscillating circuit as here shown comprises a capacitor 2C, and a winding EW mounted on a magnetizable core and connected in multiple with the capacitor 2C. The winding EW is here illustrated as the primary winding of a transformer 2TT. It will be obvious that if the oscillating circuit including the capacitor 2C, and winding EW is alternately energized and deenergized, oscillations will be set up in the oscillating circuit during the period when no energy is being supplied, which oscillations will have a frequency determined by the proportioning of the parts. We shall assume that the frequency of oscillation is 100 cycles per second.

As was pointed out earlier, the frequency of operation of the contacts of the code transmitting relay ZCTR is determined by the conditions of the signal control relay 3HR. The frequency of operation of relay ZCTR determines the frequency of operation of a code transmitting repeater relay ZCTPR which is energized over front contact a of relay 2CTR by an obvious circuit. The oscillating circuit comprising the capacitor 2C and winding EW is energized by battery LB over a back contact a of the repeater relay ZCTPR. When back contact a of relay 2CTPR is closed, direct current flows through the winding EW and energy is stored in the magnetizable core of transformer ZTT. When back contact a of relay ZCTPR opens, this stored energy is dissipated by the flow of an oscillating current with a damped wave form. If the oscillations were permitted to damp themselves completely, the wave form would be similar to that shown in Fig. 4. However, before the wave is completely damped, back contact a of relay ZCTPR again closes to recharge the oscillating circuit. Now, depending upon the condition of the flux in the core of transformer 2TT, the flux stored in the magnetizable core for the next oscillation, which flux is the algebraic sum of the flux remaining from the previous oscillation and the flux established by the charging current, will be either greater than, equal to, or less than the initial flux stored in the core. Naturally, the amplitude of the initial wave of the oscillation will vary with the amount of flux stored in the core, and this variations may be great.

To eliminate this variation, it is necessary to have the magnetic condition of the core of transformer ZTT substantially the same at the time the oscillating circuit becomes energized during each cycle. One means for accomplishing this result is by momentarily short-circuiting near the end of each series of oscillations a winding which is wound on the transformer core. When this is done, although there may be phase variations between the flux remaining from the previous oscillation and the flux established by the flow of charging current, the flux remaining from the previous oscillation will have been reduced in magnitude to such an extent that it will have substantially no effect on the energy available for the next series of oscillations.

I have shown two methods of accomplishing the dissipation of the magnetic energy stored in the core near the end of each series of oscillations. In Fig. 1, I have provided a special damping winding DW on the transformer 2TT and have made relay ZCTPR a quick-release sloW-pick-up relay. When code transmitting relay ZCTR releases, repeater relay ZCTPR, due to its quick release characteristics, releases substantially simultaneously. This causes damping winding DW to become short circuited over back contact b of relay 2CTR at substantially the same time that energization current is supplied to winding EW over back contact a of repeater relay ZCTPR. Hence, the inductance of winding EW at the instant the energizing circuit of the winding becomes closed is low and the energy stored in the magnetizable core of transformer 2TT at this time is low. When relay ZCTR subsequently picks up, relay ZCTPR will remain released because of its slow pick-up characteristic. The damping winding DW will now be open circuited because back contact b of relay 2CTR is now open, thus causing the inductance of winding EW to greatly increase. However, because slow pick-up relay ZCT PR has not yet picked up, the oscillating circuit will still be energized by the battery LB over back contact a of relay ZC'IPR, and a substantial amount of energy will be stored in the core of the transformer 2TT. When relay ZCTPR does finally pick up, the energizing circuit for the oscillating circuit is opened. Upon the opening of this circuit, oscillations are set up in the oscillating circuit causing a voltage to develop across winding EW. Due to transformer action in the transformer 2TT, a voltage is induced in an output winding OW and this voltage is supplied to the rails 5 and 6 over a circuit including front contact c of relay 2CTR. When relays 2CTR and ZCTPR subsequently release, the cycle is again repeated.

From the foregoing it will be seen that since a similar magnetic condition in the core is achieved during each cycle of operation, the initial amplitude of oscillation during each series of oscillations will be approximately the same and relatively constant operation will be achieved, the wave form being similar to that shown in Fig. 5.

In Fig. 3, I have shown a second method of achieving a substantially identical magnetic condition in the core of the transformer 2TT during each cycle of operation. The portion of the apparatus not shown is identical with Fig. 1. Here, code transmitting repeater relay ZCTPR is a slow-release and quick pick-up relay. Assuming relays ZCTR and ZCTPR are both released, current is supplied to the oscillating circuit by battery LB over an obvious circuit including back contact a of relay ZCTPR. This flow of current will cause energy to be stored in the magnetizable core. When relay ZCTR picks up, relay ZCTPR will pick up approximately simultaneously because of its quick pick-up characteristic, thereby opening the circuit energizing the oscillating circuit. With this energizing circuit opened, oscillations will be set up in the oscillating circuit and, by transformer action, an oscillating voltage is induced in winding OW, which may be applied to rails 5 and 6 over a contact of relay ZCTR. When relay ZCTR releases, relay 2CTPR will remain picked up for a short interval because of its slow release characteristic. With relay ZCTR released and relay ZCTPR picked up, winding EW is short circuited by a circuit which can be traced from the negative terminal N of battery LB, over back contact b of relay 2CTR, front contact b of relay ZCTPR, and winding EW to the negative terminal N of battery LB. With winding EW short circuited, the remaining oscillations flowing in the oscillating circuit will be rapidly damped and the amount of energy stored in the winding EW will be reduced to practically zero. When relay ZCTPR fiinally does release, energy will be once more supplied to the oscillating circuit by battery LB and the cycle is again repeated. It should be clear that each time the oscillating circuit is energized, the magnetic condition of the magnetizable core of transformer 2TT is substantially the same. Hence, the amount of energy stored in the core will be approximately the same each cycle, and the amplitude of the initial oscillation will be substantially the same each cycle, as shown in Fig. 5.

It will be clear from the above description that relay ZCTR will operate at the code rate of the coder to which its winding is connected, which connection depends on whether or not relay 3HR is energized. It will also be clear that relay ZCTR will be picked up for approximately the same amount of time that the coder contact is closed and relay ZCTR will be released for approximately the same time that the coder contact is open. It will be obvious that each time relay 2CTR is picked up, relay ZCTPR will become energized and each time relay ZCTR releases, relay ZCTPR will become deenergized. However, due to the slow release characteristic of relay ZCTPR, that relay will not immediately release upon becoming deenergized but will lag slightly. Upon energization, however, relay ZCTPR will immediately pick up. Therefore, relay ZCTPR will operate at the same code rate as relay ZCTR, but while relay ZCTR will ideally be picked up for approximately fifty percent of the time and released fifty percent of the time, relay ZCTPR will be picked up for more than fifty percent of the time and released for less than fifty percent of the time.

In each scheme, as shown in Fig. l and Fig. 3, the oscillating circuit winding EW is coupled to output winding OW by transformer action in the transformer 2TT, and output Winding OW is connected to the rails 5 and 6 over a front contact of code transmitting relay ZCTR. From the previous description and a study of the figures, it should be clear that output winding OW of the transformer 2TT is not connected to rails 5 and 6 until the oscillations of the previous cycle have been substantially damped and a new series of oscillations is ready to flow in the oscillating circuit. It has also been shown that track battery 2TB is connected to rails 5 and 6 over a back contact of relay ZCTR. Therefore, it should be clear that when the rails are energized with direct current they are not supplied with alternating current, and vice versa.

In operation, with no trains in the sections 2T and 3T, the track circuit associated with track section 2T will be energized alternately, 180 times per minute, with direct current and alternating current of 100 cycles per second frequency. The coded direct current will cause the green lamp G of Wayside signal 25 to become energized in the manner previously described, thus'indicating a clear track. If a first train enters section 2T in the direction shown by the arrow in Fig. 1, its wheels and axles will shunt the track relay ZTR causing it to release and stay released. This will cause the red lamp R to be lighted at wayside signal 28. During the half cycle that the direct current code is off, the alternating current code is on. These alternating current pulses will induce cur rents in the receivers RC mounted on the train. These induced currents selectively actuate the relays RA and RB and due to the fact that 180 code is being received, the lamp G of the cab signal will be lighted, indicating a clear track.

When the first train leaves section 2T and enters section 3T, signal control relay 3HR releases opening the circuit connecting the code transmitter r180CT to code transmitting relay ZCTR and closing the circuit connect ing the code transmitter 75CT to relay ZCTR, thus caus ing relay 2CTR to transmit the 75 code, both direct and alternating current. The 75 direct current code causes, in the manner previously described, the yellow lamp Y of wayside signal 28 to be lighted. A second train entering section 2T would, upon the engineer seeing signal 28, reduce speed.

As the second train enters the section 2T, its cab signal would go to yellow due to the selectivity of the decoding equipment in the cab. Furthermore, the second trains wheels and axles would shunt track relay ZTR causing it again to release and set signal 25 at red. As the first train vacates section 3T, signal control relay 3HR picks up and relay ZCTR commences once again to operate at a code rate of 180 times per minute. The cab signal of the second train would then go to green, but because track relay 2TR is still released, signal 25 would continue to show red. When the second train vacates sec tion 2T and enters section 3T track relay ZTR will commence operating and signal control relay 3HR will release causing section 2T to be energized with a code of 75 times per minute, thus causing wayside signal 28 to display a yellow aspect. When the second train vacates section 3T, relay 3HR picks up causing I80 direct and alternating current code to energize track section 2T and causiiig signal 23 to once again show a green or clear signa It should be clear that my invention is not confined to a system which alternately transmits alternating current code and direct current code, but it can be employed in a system in which just alternating current code is transmitted.

Although I have herein shown and described only two forms of apparatus for generating coded alternating current, it will be understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In combination with a source of direct current energy, the combination of a control circuit, an oscillatmg circuit comprising a winding mounted on a magnetizable core and a capacitor, said capacitor and wind mg being proportioned for resonance at a preselected frequency, a coder having a cyclic operation consisting of an onand an off period, means controlled by said coder for connecting the source of direct current energy to said oscillating circuit for storing energy in said magnetizable core and for disconnecting the source of energy from said oscillating circuit to cause a wave of oscillations to flow in said oscillating circuit during each period said oscillating circuit is disconnected from the energy source, means controlled by said coder for rapidly damping said oscillations prior to the restoring of energy in said magnetizable core, and means coupled to said oscillating circuit for at times connecting said oscillating circuit with said control circuit.

2. In combnation, a control circuit, a transformer comprising at least two windings mounted on a magnetizable core, an oscillating circuit including a capacitor and a first Winding of said transformer, said first winding and capacitor being proportioned for resonance at a preselected frequency, a source of direct current energy, a coder having a cyclic operation consisting of an on and an off period, a first circuit means including a contact of said coder for connecting said energy source to said oscillating circuit for storing energy in said magnetizable core and for disconnecting said energy source from said oscillating circuit to cause a wave of oscillations to fiow in said oscillating circuit during each period said oscillating circuit is disconnected from said energy source, means controlled by said coder for rapidly damping said oscillations prior to the restoring of energy in said magnetizable core, and circuit means for at times connecting a second winding of said transformer to said control circuit when said oscillations flow in said oscillating circuit.

3. In combination with a source of direct current energy, the combination of a control circuit, a first coding means provided with a first set of contacts which are operated cyclically, a second coding means operating at the same code rate as said first coding means and provided with a second set of contacts which release approximately simultaneously with said first set of contacts but which pick up after said first set of contacts, a transformer comprising three windings mounted on a magnetizable core, an oscillating circuit comprising a first winding of said transformer and a capacitor, said first winding and capacitor being proportioned for resonance at a preselected frequency, a first circuit means including a back contact of said second coding means for connecting the energy source to said oscillating circuit for storing energy in said magnetizable core, a second winding of said transformer being at times short circuited by a back contact of said first coding means for rapidly damping oscillations in said oscillating circuit, and a second circuit means including a third winding of said transformer and a front contact of said first coding means for supplying said oscillations to said control circuit.

4. In combination, a control circuit, a source of direct current energy, a code transmitting relay energized cyclically by suitable means, a repeater relay whose contacts release rapidly but pick up only after a time delay, a circuit including a first front contact of said code transmitting relay and a winding of said repeater relay for cyclically energizing said repeater relay from said energy source, a transformer comprising three windings mounted on a magnetizable core, an oscillating circuit including a capacitor and a first winding of said transformer, said capacitor and first winding being proportioned for resonance at a preselected frequency, a circuit including a back contact of said repeater relay for supplying energy from said direct current source to said oscillating circuit, a second winding of said transformer being short circuited by a back contact of said code transmitting relay for damping oscillations flowing in said oscillating circuit, and a circuit including a third winding of said transformer and a second front contact of said code transmitting relay for supplying said oscillations to said control circuit.

5. In combination with a source of direct current energy, the combination of a control circuit, a first coding means provided with a first set of contacts which are operated cylically, a second coding means operating at the same code rate as said first coding means and provided with a second set of contacts which pick up approximately simultaneously with said first set of contacts but which release after said first set of contacts releases, a transformer comprising two windings mounted on a magnetizable core, a capacitor an oscillating circuit including a first winding of said transformer and said capacitor, said first winding and capacitor being proportioned for resonance at a preselected frequency, circuit means for connecting said energy source to said oscillating circuit for storing energy in said mag netizable core including a back contact of said second coding means, circuit means for short circuiting said oscillating circuit for rapidly damping oscillations flowing in said oscillating circuit including a back contact of said first coding means and a front contact of said second coding means, and circuit means including the second winding of said transformer and a from contact of said first coding means forsupplying said oscillations to said control circuit.

6. In combination, a control circuit, a source of direct current energy, a code transmitting relay energized cyclically by suitable means, a repeater relay whose contacts pick up rapidly but release only after a time delay, a circuit including a first front contact of said code transmitting relay and a winding of said repeater relay for cyclically energizing said repeater relay from said energy source, a transformer comprising two windings mounted on a magnetizable core, an oscillating circuit including a first winding of said transformer and a capacitor, said first winding and capacitor being proportioned for resonance at a preselected frequency, a circuit for connecting said source of energy to said oscillating circuit for storing energy in said magnetizable core including a back contact of said repeater relay, a

circuit for short circuiting said oscillating circuit for rapidly damping oscillations flowing in said oscillating circuit including a back contact of said code transmitting relay and a front contact of said repeater relay, and a circuit including a second winding of siad transformer and a second front contact of said code transmitting relay for supplying said oscillations to said control circuit.

References Cited in the file of this patent UNITED STATES PATENTS 2,280,018 Agnew Apr. 14, 1942 2,331,815 Thompson Oct. 12, 1943 2,348,525 Cravath May 9, 1944 2,469,960 Gilson et a1. May 10, 1949 

